Bisphenol-A (“BPA”) is an important starting material for the production of polycarbonate plastics and epoxy resins. It is most often produced by the acid catalyzed condensation of phenol with acetone by the following reaction:2 Phenol+Acetone→Bisphenol A+Water
While a homogenous acid catalyst such as hydrogen chloride has been used for many years, heterogeneous catalyst systems have become the more popular technique for producing BPA. Most of the heterogeneous catalysts are sulfonated polystyrene ion exchange resins. These catalysts provide a number of advantages. They are non-corrosive and easily separated from the reaction mixture. The catalysts work with continuous processes such as continuous fixed bed technology which uses a fixed bed reactor. For example, a phenol/acetone mixture is continuously fed through a bed of a heterogeneous catalyst ion exchange resin and, at steady state, water is constantly generated from the reaction and removed with the continuous flow of phenol through the resin bed. The desired BPA product is then collected by crystallization and further purified by recrystallization.
A well-known drawback of this technology is that the by-product water greatly reduces the activity of the resin catalyst. This is because the water strongly hydrates the acid groups, thus competing for these sites with the reactants. As the amount of water adsorbed on the catalytic sites increases the resin activity decreases. Without some means for removal of the water, the activity of the resin catalyst becomes unacceptably slow. As a result of this inhibition, BPA reactors can require higher resin volumes adding to the cost of the BPA plant. Co-catalysts can be used to boost the reaction rate, adding to the catalyst cost. Higher temperatures can be used to achieve the desired productivity thus leading to increased by-product formation.
Over the years, researchers have tried to solve the water inhibition problem by developing reaction systems that continuously remove the reaction water from the reaction zone. U.S. Pat. No. 5,087,767 describes a reaction system in which pervaporation is used to continuously remove water from the reactor. The process utilizes a membrane (organic or ceramic) permeable to water but not to acetone, phenol, and BPA. More recently, U.S. Pat. No. 5,679,312 describes a reactive/stripping process for continuously stripping water from a BPA reactor. The reactor consists of a multi-stage distillation column. One of the drawbacks to this technology is that it requires the use of a nitrogen stream to strip the water from the reaction liquid.
The traditional simulated moving bed (“SMB”) process is one where a plurality of beds packed with solid media are used to carry out a continuous separation. The beds are connected endlessly in series with a unidirectional fluid flow through the system. Each bed is fitted with inlet and outlet ports that are switched on and off to create a simulated movement of the solid. The liquid flow combined with the simulated counter current solid movement results in the separation of components that have different affinities for the solid media. If a reaction is added to the system, a continuous reaction/separation system is created. For a general review of SMB technology, see Preparative and Production Scale Chromatography, Ganetsos, G., Barker, P. E., Eds.; Chromatographic Science Series Vol. 61; Marcel Dekker Inc.: New York, 1993; Chapters 12-13.
Recently, one group of researchers has investigated the potential for using an SMB device for carrying out the BPA reaction, which is described in The Simulated Moving-Bed Reactor for Production of Bisphenol A, Kawase, M.; Inoue, Y.; Araki, K.; Hashimoto, K.; Catalysis Today, Vol. 48, p. 199-209 (1999). Based upon laboratory adsorption and kinetic data they conducted a numerical simulation of a SMB process. Although they hypothesized that SMB could be suitable for the production of BPA, no experiments were conducted to confirm the predictions of the simulation. There is no discussion about the generation of impurities or the advantages that can be realized through use of non-traditional SMB flow configurations.
Accordingly, it is an object of an embodiment of the present invention to provide an efficient process for production of BPA. Therein water is continuously drawn off to reduce the adverse affects of water on the process reaction, and overcome disadvantages of water inhibition including its requirements for high reaction temperatures, large reactor volumes or use of co-catalysts. In another embodiment, it is a object of the invention to provide a process for producing BPA that can be used on a commercial scale.